The impact of influenza vaccination on surgical outcomes in COVID-19 positive patients: An analysis of 43,580 patients

Background Multiple recent studies suggest a possible protective effect of the influenza vaccine against severe acute respiratory coronavirus 2 (SARS-CoV-2). This effect has yet to be evaluated in surgical patients. This study utilizes a continuously updated federated electronic medical record (EMR) network (TriNetX, Cambridge, MA) to analyze the influence of the influenza vaccine against post-operative complications in SARS-CoV-2-positive patients. Methods The de-identified records of 73,341,020 patients globally were retrospectively screened. Two balanced cohorts totaling 43,580 surgical patients were assessed from January 2020-January 2021. Cohort One received the influenza vaccine six months-two weeks prior to SARS-CoV-2-positive diagnosis, while Cohort Two did not. Post-operative complications within 30, 60, 90, and 120 days of undergoing surgery were analyzed using common procedural terminology(CPT) codes. Outcomes were propensity score matched for characteristics including age, race, gender, diabetes, obesity, and smoking. Results SARS-CoV-2-positive patients receiving the influenza vaccine experienced significantly decreased risks of sepsis, deep vein thrombosis, dehiscence, acute myocardial infarction, surgical site infections, and death across multiple time points(p<0.05, Bonferroni Correction p = 0.0011). Number needed to vaccinate (NNV) was calculated for all significant and nominally significant findings. Conclusion Our analysis examines the potential protective effect of influenza vaccination in SARS-CoV-2-positive surgical patients. Limitations include this study’s retrospective nature and reliance on accuracy of medical coding. Future prospective studies are warranted to confirm our findings.


Introduction
With over 279 million cases and 5.3 million deaths globally, the current severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic continues to alter daily life [1]. Given the relative paucity of clinical information coupled with the ubiquitous spread of SARS-CoV-2, the medical community has been challenged to provide answers. Instrumental in finding solutions to combat the current pandemic is the need for reliable and accurate clinical data. One potential answer is federated electronic medical record (EMR) networks. Such technology can analyze millions of deidentified records in minutes thereby helping to guide future prospective studies during health crises, such as the current pandemic.
One aspect of clinical practice that has been deeply impacted by the COVID-19 pandemic is the field of surgery. Unsurprisingly, non-elective surgical procedures in SARS-CoV-2-postive patients have yielded poor post-operative outcomes with significantly increased morbidity and mortality when compared with SARS-CoV-2-negative patients [2,3]. Specifically, The COVID-Surg Collaborative has published an international cohort study of 1128 SARS-CoV-2 positive patients who underwent surgery, of whom 82% of deaths were attributed to postoperative pulmonary complications; significantly higher than surgical patients negative for SARS-CoV-2 [4].
Given the early indications that SARS-CoV-2-positive status is associated with increased risk for adverse post-operative outcomes, additional insight into ways to combat this effect is essential [2,3]. With the development of COVID vaccines, there is hope that postoperative outcomes may improve to pre-pandemic levels. However, even with the unprecedented surge in vaccine production, global demand has placed an inevitable strain on the limited supply and distribution around the world. Therefore, a large portion of the global population remains unvaccinated and vulnerable [5].
Recently, several studies have suggested that influenza vaccination is protective against adverse outcomes associated with SARS-CoV-2 including hospitalization, ICU admission, sepsis, stroke, and ED visits [6][7][8]. Multiple hypotheses regarding the underlying mechanism of influenza vaccination's potential protective effect against SARS-CoV-2 have been proposed, suggesting an increase in innate immune system acitvation [9][10][11][12][13][14][15]. Despite increasing evidence supporting influenza vaccine protection against SARS-CoV-2, this finding has yet to be examined in surgical patients. By retrospectively reviewing over 43,000 de-identified EMRs, this study aims to investigate and characterize the potential protective effect of up-to-date influenza vaccination against various adverse post-operative outcomes in SARS-CoV-2-positive surgical patients.

Methods
The EMRs of 73,341,020 patients aged 18-99 were retrospectively screened from January 2020-January 2021 in the TriNetX database (TriNetX Inc, Cambridge, MA) (Fig 1). TriNetX is a federated EMR network that aggregates the de-identified medical records of over 73 million patients from 56 participating healthcare organizations into a central, self-updating platform [16].
Portability and Accountability Act (HIPAA)-compliant measures put in place by the federated EMR network platform, this study was granted IRB exemption status and therefore the requirement for written consent forms was waived [16].

Inclusion criteria
Standardized Logical Observations Identifiers Names and Codes (LOINC) codes were used to identify patients who were positive for SARS-CoV-2 (LOINC: 94500-6) while undergoing surgery (LOINC: 1003143). Additionally, Current Procedural Terminology (CPT) codes were used to capture patients who had received influenza vaccination. Specifically, up-to-date influenza vaccination was defined as administration of either the trivalent live intranasal (90660) or inactivated intramuscular influenza vaccine (90653) within two weeks to six months prior to the date of surgery with a SARS-CoV-2 positive diagnosis. This timeframe was established based upon current CDC guidelines which state that full immunity is achieved two weeks from the date of influenza vaccination, with adequate levels of antibody protection lasting approximately six months prior to observation of a waning effect [17]. Any patients who were either outside of the age range or did not undergo surgery from January 2020-January 2021 while SARS-CoV-2-positive were excluded.
Using the TriNetX online database platform for real-time analyses, we performed propensity score matching to create cohorts consistent with the aforementioned criteria. Propensity score 1:1 balancing was completed via logistic regression utilizing version 3.7 of Python Software Foundation's Scikit-Learn package (Python Software Foundation, Delaware, USA). A greedy nearest neighbor matching algorithm approach was used, setting standard differences to a value of less than 0.1 to indicate appropriate matching. To eliminate record order bias, randomization of the record order in a covariate matrix occurred before matching. Baseline characteristics with a standardized mean difference between cohorts lower than 0.1 was considered well-balanced.

Outcomes assessed
Propensity score matching resulted in two equally-sized cohorts. Cohort One received influenza vaccination within two weeks to six months prior to undergoing surgery with a SARS-CoV-2 positive diagnosis whereas Cohort Two did not. Post-operative complications were then compared between the two cohorts 30, 60, 90, and 120 days after the index event. The following adverse outcomes were assessed using International Classification of Diseases-10 (ICD-10) codes and included Sepsis (A41. 9

Effect size analysis
Using the TriNetX platform's Analytics function, statistical analysis and logistical regression were performed by comparing indices and relative risks of outcomes only after the successful matching of cohorts with a p-value greater than 0.05. Outcomes for all measures were calculated using 95% confidence intervals (CIs). All p-values were two-sided and the alpha level was set at 0.05. To account for multiple hypothesis testing, post-hoc analysis using a Bonferroni correction of p = 0.0011 was calculated for the 11 adverse outcomes across 4 different time points. Outcomes with p-values less than 0.05 but greater than the Bonferroni correction of p = 0.0011 were deemed to be nominally significant and are indicated as such versus outcomes that were found to be truly significant even following Bonferroni correction (Tables 1-3, Fig  2). Risk ratio (RR) was defined in this study as the ratio of the probability of an adverse postoperative event occurring without history of up-to-date influenza vaccination versus the probability of the adverse post-operative event occurring with history of up-to-date influenza vaccination.
Subsequently, Absolute Risk Reduction (ARR), defined as the difference in risk of adverse post-operative outcomes between the influenza vaccinated group versus non-vaccinated group was calculated for each adverse outcome. The reciprocal of ARR was then obtained to determine number needed to treat, defined in this study as number needed to vaccinate (NNV), for all statistically significant variables at 30-120 days post-operatively. The NNV calculation allowed for the quantification of the average number of SARS-CoV-2-positve patients who needed to be vaccinated against influenza two weeks to six months prior to undergoing surgery in order to prevent one adverse post-operative outcome [18,19].

Results
A total of 1,435,293 SARS-CoV-2-positive surgery patients met inclusion criteria and were assigned to one of two cohorts based upon influenza vaccination status. 1,413,312 patients were not up-to-date on their influenza vaccination prior to undergoing any type of surgery whereas 21,981 were current (Table 1). Between-group factors for all propensity scoring categories were found to be significantly different (p<0.0001) prior to cohort balancing. Following propensity score matching, the differences between cohorts were no longer significant (p>0.05), with the exception of a significant difference in the number of Native Hawaiian/ Other Pacific Islander individuals (p = 0.0103) ( Table 2). After matching, 43,580 patients remained and were divided amongst two equally-sized cohorts of 21,790 patients based upon influenza vaccination status. Each group contained equivalent proportions of females to males, with 59.5% of patients being female and 40.5% being male. The mean age of the influenza vaccinated group was 58.9 years while the non-influenza vaccinated group had a mean age of 59.0 years.
When compared to SARS-CoV-2-positive patients without influenza vaccination prior to surgery, SARS-CoV-2-positive patients with influenza vaccination prior to surgery experi-

Discussion
To the best of our knowledge, this study is the first to examine the potential protective effects of influenza vaccination against post-operative complications in SARS-CoV-2-positive patients undergoing any surgical procedure. Our analysis also underscores the possible utility of a global, federated EMR network during worldwide health crises. The de-identified EMRs included in our study were evaluated for adverse outcomes at 30, 60, 90, and 120 days post-surgery (Fig 1). The upper limit of our study's timespan was set at   Fig 2a) and 90-120 days (Fig 2b) of surgery. � denotes statistical significance following Post-Hoc testing for Multiple Hypotheses using Bonferroni Correction to alpha level of p = 0.0011, �� denotes nominally significant values (p < 0.05) that failed to meet statistical significance following Bonferroni correction of p = 0.0011. 120 days in order to account for the possible presence of Post-Acute COVID-19 Syndrome (PACS). PACS is associated with increased risk of illness-related fatigue, dyspnea, inflammation, and neurologic symptoms. Given previous reports of substantial improvement 16-18 weeks after onset, our 120-day endpoint ensured capture of this phenomenon [20][21][22][23][24].
The potential protective effect of influenza vaccination against adverse outcomes associated with SARS-CoV-2 infection has been well-documented in non-surgical patients [25][26][27][28][29][30][31]. One of the first studies to demonstrate this correlation was a single-center retrospective by Yang et al. [6]. This study found a significant reduction in hospitalization and ICU admission in influenza-vaccinated patients. Shortly after, Conlon et al. supported this finding, demonstrating that SARS-CoV-2-positive patients immunized against influenza experienced decreased risks of hospitalization, mechanical ventilation, and length of stay [7]. Another key finding was put forth in a retrospective analysis that observed a significantly decreased risk of death in COVID-positive patients current on their influenza immunization [32]. Given our present finding of significant and nominally significant reductions in risk of sepsis, DVT, Acute MI, and dehiscence across all time points 30-120 days (Fig 2A), it appears that the potential protection afforded by influenza vaccination against SARS-CoV-2 may extend to surgical patients in the postoperative period. Further supporting this proposed protective effect are the additional significantly decreased risks of SSIs and death within 120 days following surgery (Fig 2B,  Table 3) SARS-CoV-2-positive surgical patients that underwent surgery are more vulnerable to adverse outcomes given the inflammatory and catabolic nature of surgery [33,34]. Specifically, post-operative patients release an exorbitant amount of pro-inflammatory cytokines along with increased levels of cortisol [35]. This systemic inflammation, when considered alongside the baseline hypercoagulable state induced by endothelial cell invasion by COVID-19, presumably compounds risk of poor post-operative outcomes in the surgical patient population [36].
Although the exact manner by which influenza vaccination confers protection against adverse post-operative outcomes in SARS-CoV-2-positive surgical patients remains undetermined, several mechanistic theories have been hypothesized [9][10][11][12][13][14][15]. The proposed mechanisms converge on the premise that the influenza vaccine appears to stimulate the body's innate immune system, thereby interfering with SARS-CoV-2 replication. One of these theories suggests that influenza immunization activates Toll-Like-Receptor-7 on cells, thereby impeding replication of single-stranded RNA viruses [11]. Alternatively, it has been proposed  Fig 3a) and 60-120 days (Fig 3b)  that influenza vaccination may prime natural killer cells, thus increasing innate immunity to combat viral antigens [14].
Another investigation demonstrated that immunization against influenza upregulates pulmonary angiotensin-converting enzyme 2 (ACE-2) receptors [10]. Downregulation of ACE-2 receptors, (observed in SARS-CoV-2-positive patients) induces pulmonary inflammation and coagulation as fewer angiotensin-II can bind to antioxidant ACE-2 receptors. As a result, the angiotensin-II instead binds to pro-inflammatory angiotensin I receptors resulting in systemic inflammation [15].
Two final postulates are notable in the context of the present study. Firstly, influenza vaccination stimulates an immunologic cascade leading to plaque stabilization, thus, decreasing risk of cardiovascular events. Vaccine-induced antibodies interact with bradykinin-2 receptors leading to increased nitric oxide production and an anti-inflammatory effect [9]. Lastly, the preparation of the trivalent influenza vaccine uses an oil-in-water squalene emulsion, known as MF59, that may induce protection against SARS-CoV-2 via host immune system stimulation [12].
Regardless of mechanism, the strong association between influenza vaccination and decreased risk of adverse post-operative outcomes in SARS-CoV-2-positive patients observed by this study merits further investigation. The hypothesis that individuals current on their influenza immunization have less baseline medical co-morbidities and risk factors for poor post-operative outcomes is supported by the "Before Matching" column in Table 2. In addition, recent literature has suggested that certain characteristics including age, gender, and BMI may dictate SARS-CoV-2 outcomes [37][38][39][40][41][42][43]. However, Table 2 also demonstrates the results of the stringent propensity score matching performed, accounting for numerous characteristics, which would have otherwise potentially acted as confounders.
By contextualizing study findings on a global scale, NNV has been an extremely valuable tool for effect size analysis throughout the SARS-CoV-2 pandemic to measure the protective effects of both influenza and coronavirus vaccines [8,44]. Specifically, our NNV calculations revealed that within 120 days, 223, 250, 323, and 182 individuals would have needed to have been current on their pre-operative influenza immunization to avoid one case of sepsis, acute MI, pneumonia, and death, respectively. Given, the potential benefits elucidated by NNV calculations, ramping up influenza vaccination in parallel with COVID-19 vaccination merits strong consideration.
Even with the unprecedented fast-tracking of multiple vaccines, the fact remains that a majority of the world is not fully vaccinated against SARS-CoV-2 [45]. Furthermore, current projections suggest that numerous countries may not receive sufficient Covid-19 vaccines for years, as partitioning continues to favor nations with the highest gross domestic products [46,47]. Given the delay of equitable access to the Covid-19 vaccine for the global community, there remains a need for preventative measures to attempt to curb disease burden in affected patients. Influenza vaccination maintains a status as a well-accepted and abundantly available option for the global community. Its low cost and predictable side effect profile make influenza vaccination a valuable provisional option to consider for individuals lacking access to COVID-19 vaccines, and may yield a benefit for individuals with predicted high risk of surgical mortality [43,48]. Influenza vaccination may also prove useful for citizens with coronavirus vaccine access, but a hesitancy to consent to novel vaccines [49,50]. Thus, pre-operative influenza vaccination may be beneficial in reducing SARS-CoV-2 morbidity and mortality in post-operative patients world-wide [51][52][53]. Furthermore, the global population may benefit from influenza vaccination as it can act to prevent a coronavirus and influenza 'twindemic' which could overwhelm healthcare resources [54]. This study is limited by its retrospective nature and reliance on the accuracy of medical coding. This study is also limited by its time window of January 2020-January 2021, prior to the widespread availability of SARS-CoV-2 vaccination, thereby prohibiting analysis of any potential synergistic or interactive effects between the influenza and COVID-19 vaccines in this patient population. Federated EMR networks lend themselves to measures of association, but not causation, thus future prospective studies are warranted to validate this study's finding that an emphasis on influenza vaccination will improve post-operative outcomes in COVIDpositive surgery patients.

Conclusion
Using a federated EMR network of over 73 million patients globally, this analysis examines the potential protective effect of influenza vaccination against adverse post-operative outcomes within 30, 60, 90, and 120 days of SARS-CoV-2-positive surgery patients. Significant findings in favor of the influenza vaccine in mitigating the risks of sepsis, acute MI, and dehiscence across all multiple time points while decreasing the risk of SSI and death by 120 days suggest a potential protective effect that merits further investigation and validation with prospective studies, such as randomized control trials.